An in situ x-ray absorption spectroscopic study of charged Li((1-z))Ni((1+z))O-2 cathode material

We have measured in situ the Ni K-edge X-ray absorption spectra of Li(1-z)N i(1+z)O2 cathode material charged in a nonaqueous cell. The material was ch arged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in Li(1-x-z)Ni(1+z)O2. We have de termined variations in the Ni-O and Ni-Ni coordination numbers, bond length s, and local disorders as well as the Ni K-edge energies as a function of L i content. We show that in the pristine state, the composition of the mater ial can be described by the formula Li0.86Ni1.14O2 (i.e., x = 0 and z = 0.1 4). That is, the material consists of Ni2+ (25%) and Ni3+ (75%) with half t he Ni2+ atoms residing in Li sites and the other half in the NiO2 slabs. Up on charging, initially Ni2+ is oxidized to Ni3+ up to a state of charge whi ch corresponds to x = 2z. Upon further charging to states corresponding to 2z < x less than or equal to 1 - z, Ni3+ is oxidized to Ni4+ with fractions being dependent on the values of x and z. Analysis of the edge energies fo r NiO, stoichiometric LiNiO2, and KNiIO6 as reference compounds for Ni2+, N i3+, and Ni4+, respectively, shows a quadratic dependence for edge energy v s. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same ran ge of oxidation states. Oxidation-state determination of Ni in Li(1-x-z)Ni( 1+z)O2 as a function of state of charge (i.e., Li content or x) on the basi s of edge energies yielded results which are in excellent agreement with ox idation state determinations made on the basis of the mole fractions for Ni 2+, Ni3+, and Ni4+ extracted from extended X-ray absorption fine structure spectra. (C) 1999 The Electrochemical Society. S0013-4651(98)05-014-9. All rights reserved.